In general, in order to provide high quality electric power by minimizing a frequency variation depending on a variation of the load, it is necessary to maintain a constant frequency by determining priorities of a plurality of energy storage devices depending on state of charges (SOCs) of each energy storage device and charging or discharging each energy storage device as necessary on a priority basis.
In the case of an energy storage device of the related art, discharging is performed if a frequency supplied to a power grid system is lower than a predetermined value. Meanwhile, charging is performed if the frequency supplied to the power grid system is higher than the predetermined value. In addition, a separate reserve power unit compares the SOCs of a plurality of energy storage devices and determines the priorities of the energy storage devices and how many energy storage devices participate in the market.
That is, in the technique of the related art, how many energy storage devices participate in the market or which one is rejected is determined by determining the priorities of the energy storage devices depending on the SOCs of the energy storage devices In addition, the energy is discharged if the frequency supplied to the power grid system decreases. Meanwhile, the energy is charged if the frequency supplied to the power grid system increases.
However, in the technique of the related art, an energy storage device may be rejected from the power grid system if its SOC exceeds another reference level.
In such a system, the rejected energy storage device may fail to respond to a frequency control signal. This may reduce a user's incentive chance, and a user may suffer from any disadvantage caused by failing to participate in the frequency control.
Therefore, it is demanded to provide a system capable of stably managing the SOCs of the energy storage devices.